1. Field of the Invention
The present invention relates to a surface acoustic wave resonator, a surface acoustic wave device, such as a filter or combined device, including the resonator, and a communication device including the resonator. More specifically, the present invention relates to a surface acoustic wave resonator using SH (Shear Horizontal) waves, a surface acoustic wave device such as a surface acoustic wave filter or a combined device, and a communication device.
2. Description of the Related Art
Surface acoustic wave devices have been widely used for band-pass filters in mobile radio communication apparatuses. Various types of surface acoustic wave devices are commercially produced for various applications. Among such surface acoustic wave devices, an SH type surface acoustic wave device including a quartz substrate and an interdigital transducer (IDT) made of Ta, W or Au is disclosed in, for example, EP0860943A2.
A method of manufacturing such a surface acoustic wave device will be described with reference to the sequential steps thereof.
First, a wafer 100 made of quartz is prepared as shown in FIG. 9A. A metal thin film 101 made of Ta is formed on the top surface of the wafer 100 by vapor deposition, sputtering, or other process, as shown in FIG. 9B. Unnecessary portions of the metallic thin film 101 are then removed by etching to form a plurality of patterns including a plurality of IDTs 101a and a plurality of reflectors 101b, as shown in FIG. 9C. As shown in FIG. 9D, a combination of IDTs 101a and reflectors 101b defining one surface acoustic wave element 102 is cut off by cutting of portions of the wafer 100 where any portion of IDTs 101a and reflectors 101b is not formed. The divided surface acoustic wave element 102 is provided and contained in a package 103 to connect an electrode 104 of the package 103 to the IDTs 101a electrically via bonding wire 105, as shown in FIG. 9E.
As described above, on a piezoelectric substrate made of quartz, a metallic film is formed by evaporating or sputtering a metal having a large mass load such as Ta, W, and Au, and then an electrode finger for defining the IDT of the surface acoustic wave device is formed by producing patterns of the metallic film by a method such as photo-etching. The resonance frequency of the surface acoustic wave device is primarily determined by the space between electrode fingers defining the IDT, the film thickness, and the width of the electrode fingers.
When the surface acoustic wave device is manufactured by the above-mentioned processes, the width of the electrode fingers and the film thicknesses are different or varied at every single wafer due to the lack of accuracy in the control of process parameters. These variations cause variations in frequency of the surface acoustic wave devices generated therein.
The inventors of the invention described and claimed in the present application discovered that, when the IDT is formed of a metal having a large density such as Ta, W, and Au, the IDT experiences a very serious problem with large variations in frequency due to variations in the width of the electrode fingers and the film thicknesses. More specifically, when the IDT is formed of Al which is common in general surface acoustic wave devices, frequency deviations are so small that the frequency deviations can be adjusted after the IDTs are formed. On the other hand, when the IDT is formed of a metal having a large density such as Ta, W, and Au, the frequency deviations are so great that the deviations cannot be adjusted after formation of the IDT. This is because a frequency dependence on a volume of the IDT becomes great in response to the density of material for the IDT. Thus, the frequency deviations becomes very large even if the variations in thickness or a width of the IDT are the same as the variations occurring with an IDT made of Al.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave resonator and a surface acoustic wave device which have a very small frequency deviation relative to a designed value regardless of the variations in the width of the electrode fingers and the film thicknesses.
According to one preferred embodiment of the present invention, a surface acoustic wave resonator includes a piezoelectric substrate and an IDT disposed on the piezoelectric substrate and made of a metal or an alloy having a higher specific weight than that of the piezoelectric material of the piezoelectric substrate so as to excite SH waves, wherein a ratio relating to an electrode finger defining the IDT, more specifically, a ratio of the electrode finger width relative to the sum of the electrode finger width and a space between adjacent electrode fingers, is from about 0.55 to about 0.85.
With this unique structure and arrangement, a speed of sound conductance of SH wave-type surface acoustic waves becomes blunt in sensitivity to the electrode finger width of the IDT.
The piezoelectric substrate is preferably a quartz substrate having Euler angles of approximately (0xc2x0, 121xc2x0 to 136xc2x0, and 87xc2x0 to 93xc2x0).
With this unique structure and arrangement, a surface acoustic wave resonator having a large electromechanical coefficient and excellent temperature characteristics is achieved.
A surface acoustic wave resonator may further include reflectors disposed on both sides of the IDT so as to sandwich the IDT therebetween. In such an arrangement, a ratio the electrode finger width relative to the sum of the electrode finger width and the space between adjacent electrode fingers, is preferably from about 0.55 to about 0.85.
With these features, a speed of sound conductance of SH wave-type surface acoustic waves becomes blunt in sensitivity to the electrode finger width of the IDT.
The surface acoustic wave resonator may be applied to a surface acoustic wave device or a communication device.
As a result of the unique structure and arrangement described above, a surface acoustic wave device or a communication device in which insertion losses in a pass band are greatly improved and variations in frequency are greatly reduced is achieved.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.